In addition to chromosomal damage, assessment of gene mutation is an important part of genotoxicity testing employed during preclinical safety testing. The Pig-a gene mutation assay is based on the loss of function of the Pig-a gene, which results in a lack of cell surface expression of specific proteins that are targeted to the surface by glycosylphosphatidylinositol (GPI) anchors. This cell surface phenotype is readily assessed by flow cytometric analysis of red blood cells. This unit describes a procedure for the collection, processing, and analysis of peripheral blood samples using materials supplied in MutaFlow® kits and a common benchtop flow cytometer.